This invention relates in general to a process for fabricating a photoconductive imaging member, and more specifically to the formation of a dual charge transport layer.
In the art of electrophotography, a photoconductive imaging member containing a photoconductive layer is imaged by first uniformly electrostatically charging the imaging surface of the imaging member. The member is then exposed to a pattern of activating electromagnetic radiation such as light which selectively dissipates the charge in the illuminated areas of the photoconductive layer while leaving behind an electrostatic latent image in the non-illuminated areas. The electrostatic latent image may then be developed to form a visible image by depositing finely divided properly charged toner particles on the surface of the photoconductive layer to form a toner image which is thereafter transferred to a receiving member and fixed thereto.
A photoconductive layer for use in xerography may be a homogeneous layer of a single material such as vitreous selenium or it may be a composite of layers containing a photoconductive imaging member and another material. One type of composite photoconductive photoreceptor used in xerography is illustrated in U.S. Pat. No. 4,265,990 which describes a photosensitive member having at least two electrically operative layers. One layer comprises a photoconductive layer which is capable of photogenerating holes and injecting the photogenerated holes into a contiguous charge transport layer. Such a photoconductive layer is often referred to as a charge generating or photogenerating layer. Generally, where the two electrically operative layers are supported on a conductive layer with the photoconductive layer capable of photogenerating holes and injecting photogenerated holes sandwiched between the contiguous charge transport layer and the supporting conductive layer, the outer surface of the charge transport layer is normally charged with uniform charges of a negative polarity and the supporting electrode is utilized as an anode. Obviously, the supporting electrode may function as a cathode when the charge transport layer is sandwiched between the electrode and a photoconductive layer which is capable of photogenerating holes and electrons and injecting the photogenerated holes into a charge transport layer when the outer surface of the photoconductive layer is charged with uniform charges of a negative polarity.
Other types of composite photoconductive imaging member employed in xerography include photoresponsive devices in which a conductive substrate or electrode is coated with optional blocking and/or adhesive layers, a charge transport layer such as a hole transport layer, and a photoconductive layer. Where the transport layer is a hole-transport layer, the outer surface of the photoconductive layer is charged negatively. These types of composite photoconductive imaging members are described in U.S. Pat. No. 4,585,884 which is incorporated herein in its entirety.
Various combinations of materials for charge generating layers and charge transport layers have been investigated. For example, the photosensitive member described in U.S. Pat. No. 4,265,990 utilizes a charge generating layer in contiguous contact with a charge transport layer comprising a polycarbonate resin and one or more of certain aromatic amine compounds. Various generating layers comprising photoconductive layers exhibiting the capability of photogeneration of holes and injection of the holes into a charge transport layer have also been investigated. Typical inorganic photoconductive materials utilized in the charge generating layer include amorphous selenium, trigonal selenium, and selenium alloys such as selenium-tellurium, selenium-tellurium-arsenic, selenium-arsenic, and mixtures thereof. The organic photoconductive materials utilized in the charge generating layer include metal free phthalocyanines, vanadyl phthalocyanines, hydroxygallium phthalocyanines, substituted and unsubstituted squaraine compounds, thiopyrylium compounds and azo and diazo dyes and pigments. The charge generation layer may comprise a homogeneous photoconductive material or particulate photoconductive material dispersed in a binder. Some examples of homogeneous and binder charge generation layer are disclosed in U.S. Pat. No. 4,265,990, the disclosure of which is incorporated herein in its entirety.
Organic photoreceptors can comprise either a single layer or a multilayer structure. The commonly used multilayered or composite structure contains at least a photogeneration layer, a charge transport layer and a conductive substrate. The photogeneration layer generally contains a photoconductive pigment and a polymeric binder. The charge transport layer contains a polymeric binder and charge transport molecules (e.g., aromatic amines, hydrazone derivatives, and the like). These organic, low ionization potential charge transport molecules as well as the polymeric binders are very sensitive to oxidative conditions arising from photochemical, electrochemical and chemical reactions. In copiers, duplicators and electronic printers, such charge transport molecules are frequently exposed to deleterious environmental conditions induced by light, charging devices (such as corotrons, dicorotrons, scorotrons and the like), electric fields, oxygen, oxidants and moisture. Undesirable chemical species are often formed during fabrication or during use in imaging processes which may react with key organic components in the charge transport layer or photogeneration layer of the photoreceptors. These unwanted chemical reactions can cause photoreceptor degradation, poor charge acceptance and cyclic instability.
Several types of reactive chemical species that are likely to be formed in the operational environment of a copier or an electronic printer include: (a) oxidants (e.g. peroxides, hydroperoxides, ozone, nitrous oxides, and the like); (b) both organic and inorganic radicals and diradicals (e.g. R.; RO2.; NO2.; OH.; and the like.); (c) ionic species having positive (e.g. aromatic amine) or negative charges; and (d) both singlet oxygen states can form through a sensitized photooxidation mechanism.
The foregoing chemical species can be generated from chemical, electrochemical and photochemical reactions as well as from the corona discharge in air by a charging device. The oxidative intermediates and their products usually degrade the surface of the photoreceptor and lead to various problems. If the surface of the photoreceptor degrades as a result of chemical and photochemical reactions, the photoreceptor surface becomes conductive (e.g. electrical charges develop and can laterally migrate) and exhibits image quality degradation Depending on the degree of damage, the photoreceptor degradation can lead to poor image quality, or even an inability of a copier or an electronic printer to produce a print.
Photosensitive members having at least two electrically operative layers are disclosed in, for example, U.S. Pat. No. 4,265,990 and U.S. Pat. No. 4,585,884 and provide excellent images when charged with a uniform electrostatic charge, exposed to a light image and thereafter developed with finely divided toner particles. However, when the charge transport layer comprises a film forming resin and one or more of certain aromatic amines, diamines and hydrazone compounds, difficulties have been encountered with these photosensitive members when they are used under certain conditions in copiers, duplicators and printers.
When photosensitive members having at least two electrically operative layers with the charge transport layer comprising an antioxidant, migration of the antioxidant in the charge generation layer can result and contributes to a significant increase in the residual voltages due to the acidic nature of the antioxidant.
Photosensitive members having a charge transport dual layer have been disclosed in U.S. Pat. No. 5,830,614 which is incorporated herein by reference in its entirety. The dual charge transport layer includes a first transport layer containing a charge-transport polymer and a second transport layer containing a charge-transport polymer having a lower weight percent of charge transporting segments than the charge-transporting polymer in the first transport layer. The resulting imaging member has greater resistance to corona effects and provides for a longer service life.
Photosensitive members having more than one charge transport layer have been disclosed in U.S. Pat. No. 6,214,514 which is incorporated herein by reference in its entirety. By using more than one charge transport layer sequentially applied, coating uniformity is achieved, raindrop effects are eliminated and curl is reduced.
While the above mentioned imaging members may be suitable for their intended purposes, there continues to be a need for improved imaging members which impart greater stability to electrophotographic imaging systems, thus improving xerographic performance (e.g. cyclic stability and charge uniformity) and the life of the photoconductive imaging member.